This invention relates to electron gun arrangements and more particularly, but not exclusively, to arrangements suitable for use in inductive output tubes (IOTs).
In electron gun assemblies used in IOTs and other types of gridded electron beam tubes, it is necessary to be able to accurately space apart the cathode at which the electron beam is generated and the electrode or electrodes located in front of the cathode to control the profile and/or density of the electron beam. The present invention seeks to provides an electron gun arrangement which permits close spacing to be maintained with accuracy between the cathode and adjacent electrode or electrodes and also provides a good mechanical construction.
According to the invention, there is provided an electron gun arrangement comprising: a vacuum envelope containing a cathode and an electrode located in front of the cathode; an electrode support mounted on a mount of low thermal expansivity; and a flexible member making a vacuum seal with the mount and with a component forming part of the vacuum envelope.
By employing the invention, those aspects of the electron gun arrangement concerned with the electrical part of the assembly and, where the arrangement is to be used in an IOT, the r.f. part of the arrangement are separated from the mechanical, vacuum seal aspect of the design. This permits the electrical and r.f. aspects of the arrangement to be optimized and also the mechanical aspects of the design to be optimized without needing to compromise one with respect to the other. The vacuum envelope is typically formed from several separate sections, some of which may provide support for parts of the electron gun and also provide means for applying electrical potentials to electrodes of the electron gun which are joined together by vacuum seals. In use, the electron gun arrangement becomes hot and components of the vacuum envelope and the gun assembly itself expand to an extent depending on the thermal expansivity of the materials used in the construction. Such an arrangement undergoes a great deal of thermal cycling during its lifetime. In accordance with the invention, a flexible member is included in the arrangement as part of the vacuum envelope to allow for thermal expansion. If all the components making up the vacuum envelope were rigid it is likely that cracks would occur at joints between them and the vacuum is destroyed. The compliance in the vacuum envelope structure afforded by the flexible member permits limited movement between components whilst maintaining vacuum integrity. Such a member need only be sufficiently flexible to enable it to accommodate the expected movement which occurs during thermal cycling and only a small amount of flexibility may be necessary in order to achieve this. Preferably, the flexible mount is of copper although other materials could be used.
As the electrode support is mounted on a mount of low thermal expansivity it ensures that very little movement occurs at the support during thermal cycling. In a preferred embodiment, the mount is of Kovar. Kovar is a U.S. registered trademark No. 337,962 identifying the source of an alloyed metal. The registration is currently owned by CRS Holdings, Inc. of Wilmington, Del. It is thus possible to maintain accurately the predetermined required distance between the electrode and the cathode. The electrode may be a control grid located closely adjacent the front surface of the cathode or could, for example, be a focus electrode. The mount is included as part of the vacuum envelope, making a vacuum seal with the flexible member but is not required to take up any movement due to thermal expansion. Thus there is effectively a decoupling between the electrical and the mechanical considerations of the arrangement. The accuracy requirements for the electrical components can be separated from maintenance of the vacuum envelope. The invention achieves this and yet provides a relatively simple arrangement in which it is not necessary to provide a completely separate structure for mounting the electrodes of the electron bun from the vacuum envelope. Thus the construction is also relatively compact.
The invention is particularly advantageous when it is incorporated in an IOT in which a high frequency resonant cavity surrounds the electron gun and the electrode support forms part of the microwave circuit. Again, the dimensions of this aspect can be optimized to achieve the desired high frequency effect without great concern being paid to how this would affect the integrity of the vacuum envelope.
Use of the invention provides a compact arrangement with a relatively small number of components which nevertheless permits optimization of both electrical/microwave properties of the device and the mechanical aspects.